Inventions relating to methods of producing seed potatoes using NFT (Nutrient Film Technique) are known and described in, for example, Japanese Laid-open Patent Application (Kokai) No. 5-284864.
In this method, the bottom of a bed for hydroponics is sloped, nutrient solution flows on the bottom of the bed from upstream to downstream such that the nutrient solution forms a thin layer, and potato plants are cultivated with the nutrient solution. This method is nothing more than the the method widely used for cultivating other crops such as tomatoes as applied to cultivation of potatoes. The open conventional method will now be described in more detail. The open upper portion of a cultivation bed is covered with a light-intercepting cover having planting holes. A support made of polyurethane foam is mounted on the stem of each potato plant with 4-14 leaves, at the site above the 2nd to 5th leaf from the root, which plant was grown by tissue culture or the like, and the support is inserted in the planting hole in the cover. The potato plants are cultivated in this state without any member between the flow of the nutrient solution and the stolons so as to form seed potatoes.
The conventional method has a problem that the potato tubers formed at the tips of the stolons are at least partially immersed in the nutrient solution because no members exist between the flow of the nutrient solution and the stolons. In the surface of the region of a potato tuber, which region contacts the nutrient solution, lenticel hypertrophy is generated. Potato tubers with lenticel hypertrophy tend to lose their water and are likely to rot due to infection with pathogens during storage, so that the storage qualities are extremely poor. Further, outer appearance of the potato tubers with lenticel hypertrophy is also very bad.
Japanese Laid-open Patent Application (Kokai) No. 5-284864 states that immersion of potato tubers in the nutrient solution is prevented by providing shallow grooves in the bottom of the container, and FIG. 6 shows the state that the potato tubers are formed above the grooves. However, it is impossible that all potato tubers be formed above the grooves as shown in this drawing, but most of the tubers are formed such that they contact the nutrient solution. FIG. 5 also shows the state that a number of tubers are formed in the air, apart from the nutrient solution. However, although some tubers existed in the air in the beginning of the growth of the tubers, most of these tubers come down with the growth and increase in the weight of the tubers and with the aging of the stolons to contact the bottom of the bed and there were substantially no tubers that existed in the air except for small tubers.
The above-described conventional method has also a problem that the number of produced seed potatoes is small because there is no means for stimulating the stolons so as to accelerate formation of tubers.
The above-described conventional method still has a problem that the size of the produced potatoes vary widely because tuberization of the stolons largely varies, that is, the timing of tuber initiation differ from one stolon to another so that the tubers which started to grow at an early stage grow to unnecessary large sizes, and the tubers which started to grow at a late stage do not grow enough.